Effects of low-molecular-weight polyols on the hydration status of the light-harvesting complex 2 from Rhodobacter sphaeroides 2.4.1

Low-molecular-weight (MW) polyols are organic osmolytes influencing water activity. We have investigated the effects of polyol molecules (glycerol and sorbitol) on the optical and triplet excitation dynamics of light-harvesting complex 2 (LH2) from Rhodobacter (Rba.) sphaeroides in buffer-detergent solutions. The resonance Raman spectroscopy demonstrated that, on increasing glycerol and sorbitol volume fractions ranging from 0 to 80% (v/v) (accompanied by the decreasing water activities), the planar and all-trans conformation of carotenoids (Crts) remained unchanged, and the bacteriochlorophyll a (BChl) Q(y) absorption intensity decreased. The B850 fluorescence amplitude elevated in the 20-80% v/v sorbitol and 20-40% v/v glycerol solution, but decreased in 80% v/v glycerol solution. The change of (3)[Crt*-BChl] interaction bands caused by (3)Crt*-BChl interaction had no obvious correlation with water activities against polyol volume fractions, which are rationalized by the water activity sensitive of C- and N-termini of protein which binding with BChls. The results suggest that Rba. sphaeroides LH2 is more sensitive to low-molecular-weight polyols compared with that of the thermophiles purple bacterium Thermochromatium (Tch.) tepidum we had investigated before. Graphic abstract

Automated summary

The study investigated the effects of low-molecular-weight polyols on the optical and triplet excitation dynamics of the light-harvesting complex 2 (LH2) from Rhodobacter (Rba.) sphaeroides. The results showed that the presence of glycerol and sorbitol influenced the conformations of carotenoids and the absorption intensity of bacteriochlorophyll a. Additionally, the sensitivity of Rba. sphaeroides LH2 to low-molecular-weight polyols was found to be higher compared to the thermophiles purple bacterium Thermochromatium (Tch.) tepidum.